1. Field of Invention
The invention relates to a device for capturing an image invariant optical speckle formed from interfering diffractive light, more particular, the optical speckle generated by passing scattered light through a small-aperture light restrictive element and the method thereof.
2. Related Art
When two highly coherent light beams overlap in the space and the optical path length difference less than the coherent length, an interference pattern is produced. Optical interference can be classified as constructive interference and destructive interference, where constructive interference produces spots with highly brightness whereas destructive interference results dark spots. Therefore, result of interference will be a spatial pattern with scattered bright and dark spots and is called speckle pattern or speckle image. Interference is related to the wavelength and the optical path length difference. When two light beams overlap in the space and the difference of optical path length between these two beams is odd integer multiple of half wavelength, destructive interference is produced. If the difference of optical path length between these two beams is integer multiple of the wavelength, constructive interference is created. Therefore, the sensitivity of the interference is half of the wavelength. The wavelength of light is fairly short, for example, wavelength range covered by visible light is from 0.4 μm to 0.7 μm. Half wavelength sensitivity is highly accurate and, therefore, interference effect is used extensively in a number of different fields.
When highly coherent light incidents on an optical rough surface, the scattered lights are produced and propagate in arbitrary directions in the space. When these scattered lights overlap in the space with the optical path difference less than the coherent length of the light, stable interference is produced. The result of the interference is an image with scattered bright and dark areas, called optical speckle.
Optical speckle was recognized as noise and considered as a ruining factor for an optical system. While optical speckle was discovered to have relation with the movement, it was used as a kind of sensing technique. Recently, unique of optical speckle was used as sensing technique for movement sensing. An U.S. Pat. No. 6,642,506 B1 (referred as patent 506 thereafter) published in 2003 discloses a one-dimensional displacement sensing in which the optical speckle pattern is captured by a device consists of a coherent light, an optical aperture, and an imaging lens. The favorite size of speckle can be obtained by using an aperture of suitable dimension in patent 506 in order to match the pixel size of the imaging sensor. Patent 506 further emphasized that the aperture has to be placed at the focal point of the imaging lens and, the optical axis of imaging lens and aperture is perpendicular to the surface, such that the height variation in the vertical direction on the surface has little effect on the speckle. Another U.S. patent number 20050024623 (referred as patent 623 thereafter) discloses an optical displacement detection method and device. A highly coherent light source is used to emit a coherent light beam, illuminates a surface and reflected by the surface, the specular reflected light is received by a sensor disposed in the direction of reflection. When the angle of reflected light makes with the surface equals to the angle of incident light makes with the surface, called specular reflection. The sensor receives the specular reflected and the scattered lights simultaneously, the scattered lights interfere with each other and produce a number of speckles on the sensor. By comparing the speckle pattern after the movement and the speckle pattern prior the movement, the direction and the amount of displacement can be determined.
Another related technique is published U.S. patent application WO2004075040 (referred as case 040 thereafter) discloses an optical signal processing method and device, which is used in an optical mouse with digital signal processing. By collecting the speckle movement signals, the relative displacement of the mouse with respect to the surface, on which the scattered light is produced, can be obtained. The mouse comprises electrical signal amplification and shaping modules, direction decision and counting modules, computer interface, a laser light source, and a sensor for receiving the laser speckle image. The speckle image received by the sensor is transferred to electric signal and send to the signal amplification and shaping modules. All of signal analyses described above are related to analyze the counts of bright and dark spots in the speckle image, which is captured by the sensor and, then, transferred to the direction of movement and the amount of displacement. The structure in case 040 is simple. But if the surface is smooth, the speckle size created will be very small, unfavorable for the analysis of bright and dark areas of speckle. Therefore, the resolution capability and sensitivity of the system will be degraded.
The specular reflection is received by the sensor in patent 623 and the received signal is then divided into DC and AC parts. DC part is smooth brightness of the reflected light, AC part is the bright and dark areas of the speckle. When the size of speckle is too small, the AC signal is difficult to recognizing and, therefore, is difficult to analyzing.
The aperture has to be placed at the focal point of the imaging lens in patent 506, which is a telecentric projection system. Although the system is insensitive to the variation of height in the vertical direction, the aperture in this structure has no capability to confine the image capturing area on the surface, however. That means this structure can not confine the angular field of view of sensor to the scattered light from the surface. The structure in patent 506 can be used to control the size of speckle but has limit capability to avoid the variation of speckle during the relative motion between surface and the imaging system.
Therefore, the key factors for accurately and correctly recognize a two-dimensional speckle pattern include adequacy of speckle size, brightness of speckle that is created by constructive interference, signal to noise ratio of the speckle pattern and, most of all, is the speckle in a speckle pattern image invariant. The image invariant optical speckle pattern is the unique one which is favorable for correct navigation and precision positioning and can be extensively used in such devices as laser mouse, finger navigation, intelligence card, three-dimensional finger print identification.